Humans developed antibiotics to fight bacterial infections. however, some antibiotics are no longer efective in killing the bacteria they once killed. how does the theory of evolution explain this?

Evolutionary theory predicted that bacterial resistance would happen. Given time, heredity and variation, any living organisms including the bacteria will evolve when a selective pressure, in this case an antibiotic is introduced. However it also gives doctors an patient some specific strategies for delaying even more widespread evolution of antibiotic resistance; these strategies include; avoid mild doses of antibiotics over long time periods, don't use antibiotics to treat viral infections, when treating bacterial infection with antibiotics, take all the pills, and also using combination of drugs to treat a bacterial infection.